Gas-liquid contactors, e.g., columns for distillation, rectification, and absorption processes, are in very extensive use at chemical plants for the separation or purification of mixtures, removal of trace ingredients, or for other purposes.
Typically, the equipment is built of a vertical cylinder having a number of trays held inside so that vapor is forced through liquid held up on the trays to effect gas-liquid contacting. Recently, offshore plants, or large-scale chemical plants constructed on floating offshore structures for natural gas liquefaction, ethylene manufacture, ammonia synthesis, urea processing and other processes, have come into practical use.
With those offshore plants it is inevitable that the structures are subjected to periodic pitching, rolling, or other oscillating motion or to static inclination by the external forces of waves, winds, tides, currents, ect. Accordingly, the gas-liquid contactors, which usually are 50 meters or more in height, can be seriously influenced by slight oscillation, leading to inadequate vapor-liquid contact due to channelling of the liquid on the trays and noncontact flow of the gas stream and hence to poor separation performance of the apparatus.
Responsible for the consequence is the construction of the contactors. The oscillating motion causes changes in the quantities of holdups on the trays from those in level operation and also invites uneven distribution of the liquid level on each tray, deep here and shallow or totally free of the liquid there.
The problems common to the existing equipment will now be more fully explained with reference to the drawing.
As a typical example of the conventional tray-type gas-liquid contactors, a distillation column is shown, in a schematic section, in FIG. 1. Inside the body 1 in the form of a vertical cylinder, there are mounted a number of trays 2. The periphery of each tray 2 is partly recessed to provide a space between itself and the inner wall surface of the body 1, and a weir 3 is attached perpendicularly to the recessed edge. The weir 3 extends downward short of the tray 2 immediately below, leaving a gap between its lower end and the lower tray while, at the same time, forming a downcomer 4 for falling liquid between itself and the inner wall surface of the body 1. Each tray 2 is formed with a multiplicity of holes 5 for the upward flow of gas therethrough. Liquid collects on each tray 2.
Gas is introduced into the lower part of the body 1, and flows upward through the holes 5 of the trays 2. Meanwhile, liquid is supplied to the uppermost tray 2 in the upper part of the body 1, and it overflows the weir 3 into the downcomer 4 to fall onto the next tray 2. As the gas from the holes 5 of the tray ascends through the liquid held up thereon, gas-liquid contact takes place.
In case of a distillation column, the material to be distilled is fed to the tray 2 at an intermediate point of the body 1, and its gaseous components flow upward and its liquid components downward onto the next tray 2.
The conventional apparatus, when installed on the ground, would present no problem. However, on a floating offshore structure which will periodically oscillate or statically incline under the action of external forces, e.g., of waves, winds, tides, currents, etc., the gas-liquid contactor will exhibit, as indicated in FIG. 2, alternate liquid level distributions (A), (B) on each tray 2 such that the liquid level near the inner wall surface on one side of the body 1 in the oscillating direction is higher than near the opposite side. As a result, near the inner wall surface on one side of the body 1, the quantity of the liquid that overflows the weir 3 of the tray 2 into the downcomer 4 is large, and the period of staying time of the liquid flowing down from the both wings of the weir close to the inner wall surface of the body 1 is short. This brings disadvantages, such as low efficiency of contact between the gas and liquid that pass countercurrent on the trays and unsatisfactory performance of the apparatus.